The present applicant is a specialist in the manufacture of stents of nickel-titanium shape memory alloy, manufactured from a raw material that is a tubular workpiece of that alloy. To make the stent matrix, the alloy tube workpiece is subjected to a laser-cutting step in which the laser cuts a multiplicity of slits in the tubular workpiece. Each slit extends through the entire wall thickness of the tube, and for the most part, the slits all have the same length and are all parallel to the longitudinal axis of the tubular workpiece. When one advances around the circumference of the tubular workpiece, crossing transversely over a multiplicity of the slits, one by one, alternate slits that one crosses are staggered, in the axial direction of the tube, by a distance that is around half the length of each slit. When such a slitted tube is slipped over a mandrel, and expanded radially, each slit opens out into a diamond-shaped aperture in the wall thickness of the tube. Looked at in another way, the creation of the slits at the same time creates struts of material that lie between adjacent slits, and the struts in the radially expanded tube emerge as zig-zag stenting rings with a characteristic strut length within any one zig-zag ring that is more or less half the length of each of the slits cut by the laser.
Where two struts, next adjacent within the circumference of a zig-zag ring, come together, we can call this a “cusp”. The cusps of each zig-zag ring are contiguous with cusps of the next adjacent stenting ring.
For enhanced flexibility of the zig-zag stent matrix, many of the “connector portions” between facing cusps of adjacent zig-zag stenting rings can be parted, to leave only a few (typically four or less) connector bridges between any two axially adjacent zig-zag stenting rings. See our WO94/17754. The surviving connector bridges have a length direction parallel to the longitudinal axis of the stent matrix.
However, where these connector bridges have been removed, there are still cusps of adjacent zig-zag stenting rings that are effectively “head to head” across the narrow gap with a cusp belonging to the adjacent zig-zag ring. When such a narrow gap is on the inside of the bend, upon bending the expanded stent (by movement of the body after the stent has been placed in the body), there is the likelihood of the two cusps head to head impacting on each other. It is common to call this “peak to peak”.
In this discussion, it is important to distinguish between the radially compact trans-luminal delivery disposition of the stent matrix (not very different from the as-cut disposition of the stent matrix, before expansion on the mandrel to diamond-shaped apertures) and the radially expanded and deployed configuration of the stent, where the struts form zig-zag rings. A head to head facing configuration of parted connector portion cusps is tolerable for the delivery procedure but to be avoided, if that is feasible, after stent deployment and radial expansion.
The present applicant has been interested in this objective for some years. For a previous proposal for improvements see its WO 01/76508, published Oct. 18, 2001. The present invention represents a fresh approach to the problem and, it is thought, a more elegant solution.
Other makers of stents have concerned themselves with the same objective. See for example US2004/0073290 A1 where (paragraph 0002) it is explained that “if adjacent rings are spaced too close together” then “interference can occur between adjacent rings on the inside of a bend”. Clearly, the idea of spacing the axially adjacent rings further apart has limited appeal, because it leaves the space between the rings unstented.
Self-expanding stents of nickel-titanium shape memory alloy are not particularly radiopaque and so are often equipped with radiopaque markers, of which one favoured material is tantalum because it is close to the nickel-titanium alloy in electrochemical potential, thereby minimising galvanic corrosion in the electrolyte of a bodily fluid.
Self-expanding stents are usually deployed by proximal withdrawal of a sheath of a catheter delivery system. To prevent the stent moving proximally with the withdrawing sheath, it is conventional to use a pushing annulus, that abuts the proximal end zone of the stent and resists any proximal movement of the stent relative to the stent delivery catheter as such. As stent performance and length go up, so does the compressive stress imposed on the end zone of the stent by the pushing annulus during withdrawal. It is important to avoid imposing on any part of the end zone a magnitude of stress higher than that of the design performance limits for that stent. The present inventor knows that one way to manage that peak stress is to build the stent so that the end zone has all its cusps touching a notional circle transverse to the longitudinal axis of the stent, so that the stress from the pushing annulus is shared equally amongst all those cusps. For an example of a stent with such an end zone, see WO 2006/047977.
EP-A-1767240 offers ways to increase the flexibility of a stent in its radially compact delivery disposition. It suggests resorting to portions not parallel to the stent length, such as struts that are curved, or bridges that are skewed to the long axis of the stent.